1. Field of the Invention
This invention relates to electrophotography and more particularly to liquid developers containing a positively-charged developer powder.
2. Description of the Prior Art
Among the developers converting electrostatic latent images formed by electrophotography or electrostatic recording into visible images, a developer comprising a developer powder dispersed in an insulating a liquid is called "liquid developer" and in case of employing liquid developers images having higher sharpness and resolution as compared with other developers are obtained and even low contrast electrostatic images can be converted into distinct visible images. Furthermore, in such case it is easy to obtain liquid developers containing a toner particle having smaller particle sizes and thus, liquid developers are suitable for obtaining continuous-tone photographs.
Conventional liquid developers are prepared by dispersing pigments or dyes, such as phthalocyanine blue, carbon black, nigrosin type dyes, etc. in an insulating liquid, but when only pigment or dyes are incorporated in an insulating liquid, they will not, in many cases, retain electric charges of the desired polarity at a high level. Accordingly, the charges are controlled, for example, by further incorporating in the insulating liquid a resin soluble therein, a charge regulating agent, etc., or by coating the surface of the pigment or dye particles using an insulating resin which can be soluble or insoluble in the insulating liquid, in combination with the pigment or dye particles.
In these liquid developers, the charge regulating agents or resins are relatively strongly adsorbed on the surface of the pigment particles within a short period of time immediately after or after the production thereof so that satisfactory dispersion and satisfactory charge of a toner are maintained. However, as the time elapses, the adsorbed material is gradually split off from the pigment particles. As a result, the developability which is obtained immediately after the preparation of a liquid developer can not be kept over a long period of time in many cases.
In order to eliminate the disadvantages described above, the use of a graft-carbon type liquid developer has been proposed. The term "Graft-Carbon" (Trademark, made by the Mitsubishi Gas Chemical Co., Ltd.) refers to pigment particles, especially carbon black particles, to the surface of which a vinyl polymer or a copolymer has been graft-copolymerized. Graft-carbon is described in detail in Nippon Gomu Kyokaishi (Journal of Rubber Association, Japan), vol. 38, January, pages 13-22; Kobunshi (High Molecular Weight Compound), vol. 17, pages 822-827; U.S. Pat. No. 3,557,040; Japanese Patent Publication No. 17284/70; etc. It is also known to use graft-carbon as a toner. See Kobunshi, vol. 14, September, pages 746, 749 (1965); Brochure published by the Mitsubishi Gas Chemical Co., Ltd., "Graft-Carbon", page 3; etc. A number of liquid developers for electrophotography have been proposed using graft-carbon based on the disclosure of the publications above. Examples of such liquid developers can be found in, for example, Japanese Patent Publication Nos. 27597/68, 19196/69, 6151/71, 6152/71, 6155/71, 6156/71, 8278/71, 6157/71, 4438/72, 4439/72, etc.
However, we have found that these graft-carbon type liquid developers are not always satisfactory. For instance, it is difficult to disperse in isoparaffin type carrier liquids the graft carbon type liquid developers as are described in Japanese Patent Publication No. 19196/69 and therefore it is necessary to employ a kneading step. The graft-carbon type liquid developers as are described in Japanese Patent Publication No. 19196/69 have a disadvantage that charges of a toner are unsatisfactory in the carrier liquid. Other liquid developers which have been proposed heretofore do not result in high optical density of images. When a latent electrostatic image produced on a sensitized zinc oxide layer was developed using these liquid developers, the maximum reflection intensity of the image obtained was 1.35 to 1.65. Since the maximum reflection intensity of original prints to be copied is usually higher than 2, it is impossible to produce a satisfactory image as far as these liquid developers are employed. In the processes as are described in the patents above, a monomer is usually employed to assist the dispersion of the toner in the carrier phase (hereafter merely referred to as a dispersing monomer) in combination with a monomer to give the toner a charge (hereafter merely referred to as a polar monomer). We have found that if a sufficient amount of charge is to be given, a large amount of polar monomer must be copolymerized and as a result the stability of the dispersion tends to be poor.
These graft-carbon developers having relatively good developability can be obtained in case that the graft-carbon is dispersed immediately after the production thereof. However, if the developers are prepared using graft-carbon which has been allowed to stand for some time after the production of graft-carbon, an unsatisfactory dispersion often occurs. When graft-carbons are stored at higher temperatures, such a phenomenon that an unstable dispersion tends to occur is often observed. Although the reason is not clear, the phenomenon is believed to occur due to the selection of polar monomers to be copolymerized with the dispersing monomer. Further, this is concerned with either the stability of or interaction between the polar monomer moieties.
In addition, we have found that images which are obtained by the use of a toner produced using these toners having polar monomer moieties offered heretofore are insufficient in weather-resistance, although it is assumed to be also due to the unstableness of the polar monomer moieties. In particular, it is seriously disadvantageous when a steel plate having thereon an electro-photographic image is used after exposure to sun-light for a long period of time.